1. Field of The invention
This invention relates in general to measuring/monitoring of intracranial pressure and pressure volume index in human patients, and more specifically to a non-invasive method for monitoring intracranial pressure and changes in intracranial pressure.
2. Description of the related art
Monitoring of intracranial pressure and pressure volume index is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies, or other conditions, that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery.
Intracranial pressure is regularly measured and monitored by means of a pressure sensor inserted through the skull into the brain. Usually a hole is drilled in the skull, and a catheter with a pressure sensor is inserted into the brain fluid. To obtain a pressure volume index, the change in intracranial pressure is monitored after a known bolus of saline solution is inserted into the cerebrospinal fluid, or after a saline solution is inserted at a known rate. This known procedure, while simple and accurate, is not suitable for long term monitoring, because an open wound must be maintained in the skull for the catheter with the pressure sensor. Antibiotics are only partially effective in treating cranial infections, so the pressure sensor can only be left in situ for two weeks or less.
Long term monitoring of intracranial pressure, without the need for maintaining an open wound in the skull, is possible if a pressure sensor with a transmitter is implanted into the brain. The intracranial pressure is thereafter monitored by means of a receiver located outside the skull. Such a solution however, is unattractive because of risks involved in implanting anything in the brain, and because of the problems of providing power to an implanted transmitter. One such remote pressure sensor is described in U.S. Pat. No. 4,124,023 to Fleischmann et al. However, this device uses nuclear material as an energy source, making it poorly suited for implantation into a human brain.
Other methods, claiming to be non-invasive methods suitable for monitoring of intracranial pressure, are based on the measurement of some quantity that depends on intracranial pressure, but which does not have a fixed relationship to intracranial pressure.
One such method is described in U.S. Pat. No. 4,204,547 to Allocca. Allocca occludes the blood flow in a jugular vein for a few seconds, and measures the resulting rate of change of blood flow within the jugular vein upstream of the occlusion as an indicator of the intracranial pressure.
Another such method, proposed in U.S. Pat. No. 4,564,022 to Bosenfeld et al., directs a sensory stimulus towards the patient, e.g. a flash of light into the eyes, and measures the latency of a resulting negative-going wave of electrical brain activity as an indicator of intracranial pressure.
These known indirect methods may be used, under very restricted conditions, as possible indicators of variations of the intracranial pressure in a patient. However, absolute values for the intracranial pressure cannot be obtained directly as there is no predetermined fixed ratio between the observed signals, obtained by these known non-invasive monitoring methods, and the absolute value of the intracranial pressure. Such calibration is possible by inserting a pressure sensor into the brain of the patient being monitored, however, this is a traumatic and undesirable procedure.